Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of a clean energy technology.
There are many different solar cell architectures. Two specific designs are the selective emitter (SE) and the interdigitated backside contact (IBC). A SE solar cell has high-dose regions across the lightly doped surface to enable better current generation in the lightly doped regions while enabling low-resistance contacts for current collection in the high-dose regions. An IBC solar cell has alternating p-type and n-type regions across the surface not impinged by sunlight. Both a SE and IBC solar cell may be implanted to dope the various n-type or p-type regions.
“Glitches” may occur during implantation. A glitch is when the beam quality is suddenly degraded in the middle of an implantation operation, potentially rendering the workpiece unusable. Such a glitch can be caused at various locations along the beam path. Ion implanters generally employ several electrodes along the beam path, which accelerate the beam, decelerate the beam, or suppress spurious streams of electrons that are generated during operation. Generally, glitches occur across acceleration gaps, deceleration gaps, or suppression gaps, although glitches may occur elsewhere. These glitches may be detected as a sharp change in the current from one of the power supply units for the electrodes. This causes a change in the delivered ion dose to the workpiece surface. Due to the threat of potential impact to the performance of the workpiece being implanted, glitches can be quite costly. Thus, steps are usually taken to both minimize the occurrence of such glitches and to recover from the glitches if possible.
When a glitch is detected, one solution is to immediately reduce the ion beam current to zero, thus terminating the implantation at a defined location on the workpiece. Once the glitch condition has been removed, implantation ideally resumes at exactly the same location on the workpiece with ideally the same beam characteristics that existed when the glitch was detected. The goal is to achieve a uniform doping profile, and this can be achieved by controlling the beam current, the workpiece scan speed, or the workpiece exposure time. Repairing the dose loss caused by the glitch in such a manner may take over 30 seconds, which may be too time-consuming for the throughput demands of certain workpiece manufacturing industries, such as the solar cell industry. Therefore, there is a need in the art for an improved method of glitch recovery for the implantation of workpieces such as solar cells.